What is pitting corrosion and how does it happen?

The composition of duplex stainless steel is what sets it apart from regular austenitic and ferritic grades when comparing corrosion resistance. Duplex stainless steels are less prone to Stress Corrosion Cracking (SCC) than austenitic steels like 304 and 316. Although there is no single measure of corrosion resistance, to rank the different grades, the Pitting Resistance Equivalent Number (PREN) method is often used.

Pitting corrosion is in itself a corrosion mechanism, but is also a form of corrosion often associated with other types of corrosion mechanisms.

It is characterized by a highly localized loss of metal. In the extreme case, it can appear as a deep, tiny hole in an otherwise unaffected surface. The adjacent figure illustrates the nature of pitting and other forms of corrosion.

The initiation of a pit is associated with the breakdown of the protective film on the metal surface. In cases where pit depths increase rapidly, the environment is usually such that no repair or repassivation of the protective layer can be accomplished. For other instances where many shallow pits form, the environment is usually one where repassivation of the damaged film can be made, but initiation of new sites is occurring on a regular basis.

The localized nature of pitting attack can be associated with component geometry, the mechanics of the corrosion process, or with imperfections in the material itself. The growth of pits, once initiated, is closely related to another corrosion mechanism, crevice corrosion.

In stainless and nickel alloys, chloride ions are particularly efficient at breaking down the passive protective film. Experience has shown that alloy additions of chromium and molybdenum are particularly beneficial in increasing resistance to pitting attack. Laboratory testing has shown a correlation that has been identified as the Pitting Resistance Equivalent (PRE) where PRE = %Cr + 3.3 * %Mo + 16 * %N. Increasing resistance to pitting (and crevice corrosion) is found as the PRE increases. Typical PREs for several alloys are shown in the table below:

Alloy PRE
304L 19.5
316L 23.5
2205 35.0
ZERON 100 40.1
AL-6XN 43.2
625 49.5

That’s the reson why duplex stainless is replacing conventional stainless steel, to learn more about the duplex stainless steel grades, click the link below: